A standard bicycle drive train comprises a driving axle journalled for rotation in the frame of a bicycle. A crank arm is mounted at each end of the driving axle. The two crank arms extend at right angles to the driving axle, in opposite directions. A pedal extends outwardly from the end of each crank arm. A rider pedals the bicycle with his or her feet to cause the driving axle to rotate about its axis. The pedals traverse circular trajectories about the driving axle with the sense of rotation such that each pedal is travelling toward the front of the bicycle when it is at the top of its path. The rear wheel of the bicycle is driven by a roller chain which passes around a drive sprocket mounted to the driving axle and around a driven sprocket connected to the rear wheel of the bicycle.
The standard bicycle drive train is simple and reasonably effective. Because bicycles are relatively inexpensive, and widely available, and because a bicycle drive train harnesses the leg muscles which are among the strongest muscles in the human body, bicycle-type drive trains are used in various applications. For example, bicycle-type drive trains may be used to power water pumps, to drive small generators, or to provide a source of mechanical power for operating various types of machinery in places where electrical power is not readily available.
There are several problems with standard bicycle drive trains. One problem is that a bicycle drive train, as described above, is not well matched to the capabilities of the human body. In a standard bicycle drive-train, the maximum torque which a rider can deliver varies with the angular position of the bicycle pedals. The result is a pulsating flow of power to the driven sprocket. This is especially true at low rates of rotation and high torque load such as may occur when a bicycle is being ridden up a hill. Ideally, constant torque would be applied to the driven sprocket for on-road use. In off-road conditions a moderate level of pulsation can improve tire grip.
Each revolution of a bicycle pedal can be divided roughly into four phases. First, the rider kicks forward when the pedal is between the ten o'clock position and the one o'clock position. Second, the rider pushes downward as the pedal moves from the one o'clock position to the five o'clock position. This second portion of the cycle is termed the "power stroke". Third, the rider drags or paws the pedal from the five o'clock position to the seven o'clock position. Finally, the rider lifts his or her foot as the pedal moves from its seven o'clock position back to its ten o'clock position. Due to the skeletal structure and arrangement of muscle groups in a typical human body, the power stroke is the most powerful and efficient stroke. The combined kicking and dragging motions of both legs generate substantially less torque. The lifting motion is relatively ineffective at generating power. However, this is compensated for by the fact that while one leg is being lifted the other pedal is in its power stroke. The forces which a rider is capable of exerting on the pedals also depend upon the position of the rider's legs and body relative to the pedals.
A rider is generally able to exert a maximum torque on the driving axle during the portion of the power stroke when one of the two crank arms is extended horizontally in front of the driving axle (i.e. when the crank arm is in its "3 o'clock" position which is 90.degree. clockwise from the pedal's top dead centre position when viewed from the right-hand side of the bicycle). At this point the rider is able to apply his or her full weight in a direction perpendicular to the crank arm and is able to create more torque about the driving axle than is possible when the crank arm is in other positions.
The torque which the rider can deliver to the driving axle is the sum of the torques about the axle which result from the forces exerted by the rider on each of the two pedals. As a result, when a rider is attempting to maximize the energy delivered to the rear wheel of the bicycle, the torque on the driving axle rises and falls twice during each rotation of the pedals. This causes the bicycle to accelerate and decelerate. The slight increase in speed resulting from each power pulse is generally wasted to wind drag and rolling friction during the slack period when the rider is generating a reduced amount of torque.
A second problem with a standard bicycle drive-train is its tendency to stall at low rates of rotation and high torque loads. When one crank arm points vertically downward, the opposite side crank arm points vertically upward. In this configuration it is difficult for the rider to start the pedals moving or to accelerate the rotation of the pedals because neither pedal is in a position which allows the rider to apply much torque to the driving axle.
A third problem which is inherent in the construction of a standard bicycle drive train is that when a rider is attempting to exert maximum torque on the bicycle's drive-train the rider tends to bob up and down and from side to side as he or she alternately applies force to the left and right pedals of the bicycle. This tends to keep the rider slightly off balance.
A fourth problem with a standard bicycle drive train is that the rider is forced to shift his or her weight slightly forward and rearward twice during each full rotation of the pedals. When the rider is standing and attempting to apply maximum torque to a standard bicycle drive train, as one pedal passes 90.degree., virtually all of the rider's weight is on that forward pedal. If the rider is balanced, the rider's centre of gravity is above the forward pedal. When the crank arms are both vertical, as happens twice during each full rotation of the pedals, the rider's centre of gravity is directly above the driving axle. Therefore, the rider's centre of gravity moves fore and aft a distance equivalent to the length of one of the crank arms twice during each full rotation of the pedals. This unnecessary motion can put the rider off balance and can result in the rider wasting energy.
A fifth problem with a standard bicycle drive train is that at any given time, only one of the rider's two legs is in the power stroke portion of its cycle. Assuming that the pedals rotate at a constant rate, each of the two crank arms spends, on average, only about one third of each cycle in the power stroke portion of its cycle. Each of the rider's legs spends the majority of each cycle in a position where it is not capable of applying a significant amount of torque about the driving axle.
A sixth problem which can occur with a standard bicycle drive train is that the rider can tend to "bounce". This happens because the geometry of a standard bicycle drive train is such that the rider's left leg may not reach its highest point at exactly the same time as the rider's right leg is at its lowest point and vice versa. The rider's upper legs pivot up and down at the rider's hips as the rider pedals. When the rider's right leg is at its lowermost position (i.e. if the bicycle is properly fitted to the rider when the right pedal is near its bottom dead centre position) the rider's left leg has not yet reached its highest point. Because of the geometry of the human body, the rider's left knee is typically not fully raised until after the left pedal is approximately 15.degree. past its top dead centre position. This asymmetry causes a net upward force on the rider's torso twice in each full revolution of the bicycle's pedals. At high rates of pedal rotation the bounce can be bad enough to make it hard for the rider to remain firmly seated on the bicycle.
Various prior art mechanisms have been proposed to enable a human rider to deliver constant torque to a bicycle drive system throughout a complete revolution of the bicycle pedals. Such systems include those described in Yamaguchi U.S. Pat. No. 4,560,182; Vereyken U.S. Pat. No. 4,577,879; Trammell Jr., U.S. Pat. Nos. 4,029,334, 3,779,099, 3,906,807; Cropek U.S. Pat. No. 4,898,047; Pontin, U.S. Pat. No. 3,132,877 and Phillips U.S. Pat. No. RE11,331; and, the BIOPACE.TM. system manufactured by Shimano American Corporation of Irvine, Calif. The BIOPACE.TM. system provides a mechanical advantage which varies with the position of the bicycle pedals by using an oval shaped drive sprocket.